1. Field of the Invention
The technical field relates to a semiconductor device, a display device, a light-emitting device, and a method for manufacturing these devices. The technical field particularly relates to a semiconductor device including a thin film transistor (hereinafter also referred to as a TFT) using an oxide semiconductor.
2. Description of the Related Art
In recent years, thin film transistors (TFTs) in which a silicon layer of amorphous silicon or the like is used as a channel layer have been widely used as switching elements in display devices typified by liquid crystal display devices. Although the field-effect mobility is low, a thin film transistor using amorphous silicon has an advantage in responding to increase in size of glass substrate.
Moreover, attention has been recently drawn to a technique by which a thin film transistor is manufactured using a metal oxide with semiconductor characteristics and such a transistor is applied to an electronic device or an optical device. For example, it is known that some metal oxides such as tungsten oxide, tin oxide, indium oxide, and zinc oxide have semiconductor characteristics. A thin film transistor in which a transparent semiconductor layer formed of such a metal oxide is used as a channel formation region is disclosed (e.g., see Patent Document 1).
Furthermore, a technique has been considered to increase the aperture ratio in such a manner that a channel layer of a transistor is formed of a light-transmitting oxide semiconductor layer and a gate electrode, a source electrode, and a drain electrode are formed of a transparent conductive film with a light-transmitting property (e.g., see Patent Document 2).
Increase in aperture ratio increases the light use efficiency, and reduction in power and size of a display device can be achieved. Meanwhile, in terms of increase in size and application to portable devices of display devices, further reduction in power consumption as well as increase in aperture ratio is required.
As a method for placing a metal auxiliary wiring for a transparent electrode of an electro-optic element, a method is known in which a metal auxiliary wiring and a transparent electrode are placed to overlap with each other so that the auxiliary wiring is brought into conduction with the transparent electrode above or below the transparent electrode (e.g., see Patent Document 3).
A structure is known in which an additional capacitance electrode provided for an active matrix substrate is formed of a transparent conductive film of ITO, SnO2, or the like and an auxiliary wiring formed of a metal film is provided in contact with the additional capacitance electrode in order to reduce the electric resistance of the additional capacitance electrode (e.g., see Patent Document 4).
It is known that in an electric-field transistor using an amorphous oxide semiconductor film, a transparent electrode formed of indium tin oxide (ITO), indium zinc oxide, ZnO, SnO2, or the like; a metal electrode formed of Al, Ag, Cr, Ni, Mo, Au, Ti, Ta, or the like; a metal electrode formed of an alloy containing any of the above elements; or the like can be used for a gate electrode, a source electrode, and a drain electrode, and two or more of such materials may be stacked to reduce the contact resistance or to increase the interface intensity (e.g., see Patent Document 5).
It is known that a metal such as indium (In), aluminum (Al), gold (Au), or silver (Ag); or an oxide material such as indium oxide (In2O3), tin oxide (SnO2), zinc oxide (ZnO), cadmium oxide (CdO), indium cadmium oxide (CdIn2O4), cadmium tin oxide (Cd2SnO4), or zinc tin oxide (Zn2SnO4) can be used as materials for a source electrode, a drain electrode, and a gate electrode of a transistor using an amorphous oxide semiconductor and an auxiliary capacitance electrode, and the materials for the gate electrode, the source electrode, and the drain electrode may be the same or different from each other (e.g., see Patent Documents 6 and 7).